In the past two decades, tobacco expansion processes have become an important part of the cigarette manufacturing process. Tobacco expansion processes are used to restore tobacco bulk density and/or volume which are lost during curing and storage of tobacco leaf. In addition, expanded tobacco is an important component of many low tar and ultra-low tar cigarettes.
Commercially significant tobacco expansion processes are described in U.S. Pat. No. 3,524,451 to Fredrickson and U.S. Pat. No. 3,524,452 to Moser et al. These patents describe processes in which tobacco is contacted with an impregnant and then heated rapidly to volatilize the impregnant and expand the tobacco. A variation of these processes is described in U.S. Pat. No. 3,683,937 to Fredrickson et al. which discloses a tobacco expansion process employing an organic compound in the vapor state for impregnating tobacco. The impregnated tobacco is expanded either by heating or rapidly reducing pressure.
The use of a carbon dioxide for expanding tobacco is disclosed in U.S. Pat. No. 4,235,250 to Utsch; U.S. Pat. No. 4,258,729 to Burde et al.; and U.S. Pat. No. 4,336,814 to Sykes et al., among others. In these and related processes, carbon dioxide, either in gas or liquid form, is contacted with tobacco for impregnation and thereafter the impregnated tobacco is subjected to rapid heating conditions to volatilize the carbon dioxide and thereby expand the tobacco. In the known carbon dioxide expansion processes, it is typically necessary to heat the tobacco excessively in order to achieve substantial and stable expansion of the tobacco. This excessive heating can harm the tobacco flavor and/or generate an excessive amount of tobacco fines. In addition, those processes which use liquid carbon dioxide for impregnating tobacco typically result in impregnated tobacco in the form of solid blocks of tobacco containing dry ice, which must be broken up prior to heat treatment, thereby increasing the complexity of the process.
U.S. Pat. No. 4,461,310 to Zeihn and U.S. Pat. No. 4,289,148 to Zeihn describe the expansion of tobacco employing supercritical nitrogen or argon impregnation of tobacco. These gases are removed from the tobacco during a rapid pressure reduction, and the tobacco is expanded by exposure to heated gas or microwave. These processes require treatment of tobacco at pressures in excess of 2,000 or 4,000 psi up to above 10,000 psi in order to achieve substantial tobacco expansion.
U.S. Pat. No. 4,531,529 to White and Conrad describes a process for increasing the filling capacity of tobacco, wherein the tobacco is impregnated with a low-boiling and highly volatile expansion agent, such as a normally gaseous halocarbon or hydrocarbon at process conditions above or near the critical pressure and temperature of the expansion agent. The pressure is quickly released to the atmosphere so that the tobacco expands without the necessity of a heating step to either expand the tobacco or fix the tobacco in the expanded condition. The pressure conditions of this process range from 36 Kg/cm.sup.2 (512 psi) and higher with no known upper limit. Pressures below 142 Kg/cm.sup.2 (2,000 psi) were used to produce satisfactory tobacco expansion without excessive fracturing. Pressures above this range were said to normally not be needed. When the time period used to increase the expansion agent pressure to the necessary pressure ranged from 1 to 10 minutes, little or no additional holding time under pressure was needed in order to achieve effective impregnation of the tobacco.
U.S. Pat. No. 4,554,932 to Conrad and White describes a fluid pressure treating apparatus, including a cylindrical tubular shell and a reciprocal spool assembly mounted for movement between a loading position outside the shell and a treating position within the shell. Sealing members on the spool assembly are provided for engaging the shell to form a pressure chamber. Conduits are provided to introduce processing fluids into the pressure chamber. This system thereby provided an apparatus for use in high pressure materials treatment, such as tobacco impregnation for expansion, permitting easy loading and unloading and minimizing or eliminating problems associated with sealing and locking mechanisms normally used in high pressure treatment apparatus. Accordingly, this apparatus provided a pressure vessel producing time savings and improving economics in tobacco expansion.
U.S. Pat. No. 5,067,293 to Kramer is directed to a process and apparatus for the treatment of tobacco material and other biological materials having a mechanism for forming a dynamic seal in which cooperating moving surfaces seal a treatment chamber. The dynamic seal system provided according to this patent is useful in treating tobacco at elevated temperature and pressure conditions, including conditions of supercritical temperature and pressure for processes including tobacco expansion. Both continual and batch processes are disclosed. For tobacco expansion the use of supercritical fluids at weight ratios relative to the tobacco, of greater than 40:1 is disclosed, and complete impregnation of the tobacco material was said to be virtually instantaneous. Greater tobacco expansion was said to be obtained when impregnation times of 1 to 10 minutes were maintained prior to depressurization.
U.S. Pat. No. 4,962,773 to White et al. describes a process for subjecting a cigarette rod to conditions such that the cut filler undergoes volume expansion while within the paper wrap. The use of various impregnation conditions and fluids is described in this patent, including the use of impregnation conditions conducted above supercritical pressure and temperature. A pressure vessel having a volume of 4.5 liters was employed in the working examples to impregnate the tobacco rods under supercritical conditions.
Tobacco expansion processes including those described above and others, must be conducted in a batch process or continual process (Kramer U.S. Pat. No. 5,067,293) when impregnation pressures substantially above atmospheric pressure are used. The batch and continual treating processes require complicated treating apparatus and increased cycle times because of the time required in opening and closing the vessels and introducing and removing impregnating agent from the vessels. Some throughput improvements have been made by modifying the various apparatus employed to decrease cycle time; however, substantial throughput improvements in the known batch systems are available according to conventional techniques primarily by increasing volumes of the individual systems and/or increasing the number of batch systems used simultaneously.